1, Claim observer-keypair (write public key to blockchain) by the signature of account. User can off/change observer-key also.
2, Titan transfer for observer mode
"public key of observer-keypair" + "private key of random keypair" ==> public key of sub-address
"public key of sub-address" + "public key of account" ==> mutisig-address
then send money to mutisig-address.
3, I can tell my private key of observer-keypair to the manager of wallet (like blockchain.info).
So they can :
"private key of observer-keypair" + "public key of random keypair" ==> public key of sub-address
"public key of sub-address" + "public key of account" ==> mutisig-address
then tell me my balance of my account( relate with the observer-keypair).
They can not spend it because they have not the private key of account.
4, I can off/change my observer-keypair to take back the right of Observer (future).